Plasticized polyvinyl butyral (PVB) sheet is used in the manufacture of laminate structures such as, for example: windshields for vehicles including automobiles, motorcycles, boats and airplanes; homes and buildings; shelving in cabinets and display cases; and other articles where structural strength is desirable in a glass sheet.
Laminated safety glass comprises a sandwich of polyvinyl acetal sheeting, typically polyvinyl butyral sheeting, between sheets (lites) of glass. These laminated glass composites are required to perform to stringent requirements including good impact performance, weatherability, and transparency.
A recent design trend toward the use of laminated safety glass in open edge windshield applications and in automobile sidelites has necessitated the need for laminated glass with improved adhesion robustness. In these open edge applications, the edge of the windshield is not encased with a gasket, but is exposed to the environment. Exposure of a PVB interlayer to the environment can result in moisture being absorbed into the interlayer. Moisture absorbed into the interlayer can affect the adhesion of the interlayer to the glass, and thereby cause defects in the laminate.
Japanese Kokai Publication Hei 7-172878 describes an interlayer with alternating sheets of specialized PVB constructed in order to improve the moisture robustness of the laminate. Japanese Kokai Publication 60-210551 describes the use of silicon oils in PVB sheeting, and Japanese Kokai Publication Hei 7[1995]-237943 describes the use of organic acids in PVB sheeting to improve moisture robustness.
The use of magnesium alone as an adhesion control agent is well known. For example, WO 9961243 describes the use of magnesium salts alone as adhesion control agents. However, one problem with using magnesium alone as an adhesion control agent is that it can yield asymmetric adhesion depending upon the structure of the glass laminate. U.S. Pat. No. 6,383,647 describes adhesion control agents. U.S. Pat. No. 4,292,372 describe the use of potassium and magnesium salts as adhesion control agents for PVB. U.S. Pat. Nos. 3,249,490 and 3,249,489 describe the use of mixtures of alkali and alkaline earth metal salts as adhesion control agents. However, the Applicant has discovered that the ratio of potassium ions to magnesium ions present in a PVB composition is an important parameter for improving adhesion robustness in a PVB laminate. The importance of the ratio of potassium ions to magnesium ions and the effect of the ratio of said salts on adhesion robustness in a PVB laminate has not been fully appreciated or described in the art.
It would be desirable to have improved control over adhesion robustness of PVB to glass in a laminate by controlling the ratio of potassium ions to magnesium ions.